processes of the arachnoid called ligamentum denticulatum,' and the nerve-roots.

§ 204. Encephalon, its primary divisions.-The encephalon, or brain, of Mammals,

like that of lower Vertebrates, Turtle, fig. 45 (vol. i., Shark, fig. 187,

45

P

R

and Lepidosiren,

fig. 186), presents

α

a

Brain of a Turtle (Chelone), side view.

four primary seg

ments or divisions, indicated by as many superincumbent, originally vesicular, masses, or pairs of masses; but consisting, not only of those, but of tracts of the myelencephalic columns from which those masses are successively developed.

'tuber annulare' or 'pons varolii,' p; the three parts, so named in anthropotomy, are subordinate elements of one and the same primary division of the encephalon.'

The next division includes the parts of the myelencephalic columns which support, and from which are developed, the optic lobes, o: it is the 'mesencephalon,' figs. 45, 46 and 47,0. With the columnar elements are the parts called the 'fillet,' and 'processus a cerebello ad testes' in anthropotomy, including the 'third ventricle' and its prolongations into the vascular appendages

The severance of the 'pons,' and raising it, in association with parts of another segment, to the rank of a distinct primary division as mesocephalon,' and the severance of the medulla oblongata' from the cerebellum, as a co-equal division, called 'metencephalon,' indicate the warping of the judgment through habitual contemplation of the characteristically modified and developed parts of the human brain.

called 'pineal' and 'pituitary' h, glands: a second pair of ganglionic masses are developed in Mammalia behind the optic lobes, o, and received from the old anthropotomists the name of testes,' the more constant and important pair being the 'nates,' and the whole, from their arrested condition in Man, forming the corpora 'quadrigemina' or bigemina.'

6

The third primary division of the brain includes the crura cerebri' with the reinforcing or recruiting ganglions called 'thalami optici' and 'corpora striata,' and the superincumbent masses called cerebral hemispheres: it is the 'prosencephalon,' figs. 46 and 47, p.

The foremost primary division of the brain includes the anterior termination of the columnar tracts, called crura rhinencephali,' and the appended vesicular mass, called olfactory lobe;' it is the 'rhinencephalon,' ib. R. The nature and value of this division are masked, in Man, by the arrest of its developement and the contrast of the excessive expansion of the vesicular part of the antecedent division. Accordingly the crura rhinencephali' are termed olfactory nerve' with its roots,' and the primary vesicle is the bulb of the olfactory nerve,' of anthropotomy.

Each primary encephalic division has its cavity or cavities called ventricles.' The epencephalic prolongation of the myelonal canal is the fourth ventricle:' its continuation into the primary vesicle is the cerebellar ventricle' it is persistent in fishes (vol. i. p. 275, fig. 178, c), reptiles (ib. p. 295, fig. 193), and birds (vol. ii. p. 120, fig, 45), but is obliterated in Mammals where the cerebellum is solid. The myelonal canal' passes forward as the third ventricle,' and iter' or communicating canal between that and the fourth.' Its continuation into the optic lobes, retained in oviparous Vertebrates (vol. i. p. 278, fig. 182, h, b, p. 279, fig. 183, d, p. 295, fig. 193, 3, vol. ii. p. 120, fig. 45, o,) is obliterated by growth of neurine in Mammals; as is also its ascending canal to the pineal appendage;' the descending one to the hypophysis' is retained as the infundibulum.'

6

Each cerebral hemisphere begins in Mammals, as in lower Vertebrates, as a bladder with a thin wall of brain-substance, the cavity including, potentially, all the anthropotomical 'horns,' 'fore,' 'aft,' and 'under,' of the 'lateral ventricle,' which are subsequently meted out by endogenous growths of grey and white neurine, in size and shape according to the group or genus.

In most Mammals which derive so important a share of their ideas through the olfactory sense, the lateral ventricle' is con

So

tinued into the rhinencephalon,' as shown in fig. 46, d. that all the essential parts of a primary encephalic division, viz. the columnar as 'crus rhinencephali,' the superimposed mass, and the cavity exemplifying the nature of the olfactory bulb as a 'primary vesicle' of the brain, are present.

§ 205. Macromyelon.-The epencephalon consists of the macromyelon and cerebellum. The term 'macromyelon' is not exactly the equivalent of the medulla oblongata' of anthropotomy, the authorities in that department of anatomy having applied the phrase in different senses. With Willis,' it included the part of the brain beneath the cerebellum and cerebral hemispheres, all that substance,' e.g., which reaches from the cavity of the callous body and conjuncture in the basis of the head to the hole at the hinder part where the same substance, being further continued, ends in the spinal marrow.' With Vieussens, the oblong marrow' included the columns of the neural axis between the 'spinal marrow' and the cerebral hemispheres,' with the crura cerebri' and their ganglionic enlargements, called 'optic thalami,' and corpora striata.' Winslow 3 defines the medulla oblongata' as the medullary basis common to both cerebrum and cerebellum. Haller restricts the medulla oblongata' to the intracranial myelonal columns, as far as the 'pons varolii.' Rolando prefers the older view of its extent. Chaussier, again, distinguishes the portions of the intracranial columns crossed by the transverse commissural fibres of the cerebellum as a primary division of the brain, under the name mesocephale;' and this term has been extended by Todd' to include the corpora quadrigemina' with the processus cerebelli ad testes,' and part of the floor of the fourth ventricle.

But the developement of the human brain and its several stages, represented by the conditions at which it is arrested in lower vertebrates, show that the transverse commissural fibres which cross or decussate with the intracranial myelonal columns, whether under the name of 'pons,' or trapezoid bodies,' or 'arciform fibres,' are subordinate adjuncts to other parts, chiefly the cerebellum; while the distinct and superimposed masses called 'corpora quadrigemina' include the true correlatives of the cerebrum and cerebellum, as primary vesicles of the brain.

By macromyelon,' therefore, I signify the intracranial prolongations of the myelonal columns as far forward as their emergence from the pons,' or cerebellar commissure: in this tract they are

reinforced by masses of grey neurine, and the transverse commissural fibres are so intermixed with the longitudinal ones as to compel their being combined in description as in delineation, figs. 48, 56. But, before quitting the Mammalian class, the reduction of the 'pons,' concomitantly with that of the side-lobes of the cerebellum, as in figs. 51 and 53, is such as significantly to testify against its title to be regarded as a primary division of the brain; and in birds a tuber annulare' or 'pons varolii,' ceases to appear upon the under surface of the myelencephalous tract above defined. From this tract the cerebral nerves, from the fifth to the hypoglossal or ninth inclusive, arise.

G

V

In advancing to the formation of the macromyelon growing

48

central tracts of the myelonal columns come to the periphery, and push aside the medial tracts on both the ventral and dorsal surfaces. On the former, fig. 48, they decussate, as they appear, at d, and, with a contiguous portion of the anterior myelonal columns, b, expand to form the prepyramidal bodies,' p. The rest of the anterior columns, b, with the contiguous antero-lateral lumn, in their course along the macromyelon, are associated with a mass of grey matter occasioning a swelling out of the surface, called the olivary bodies,' ib. o. A thin layer of superficial fibres which, in lower Mammals with non-prominent 'olives' pass outward, as a trapezoid layer,' in Man curve round the exterior of the olivary prominences, and constitute the arciform fibres,' ib. A.

Macromyelon, anterior or ventral aspect.
Man, nat. size.

The transverse fibres defining anteriorly the 'prepyramids and olives' increase in mass, from the lowest Mammals (Ornithorhynchus, fig. 51, c, Didelphys, fig. 53, b), to Man, fig. 48, a. As they arch over the fore part of those macromyelonal tracts they have been called 'pons;' but their true position is that of an inverted or suspended bridge: their developement is in the ratio of that of the side-lobes of the cerebellum.

On the posterior or dorsal surface of the myelon the deep

seated tracts become superficial at a greater distance from the skull than on the ventral surface, and do not decussate; they expand as they enter the macromyelon, and form the 'post-pyramidal bodies,' fig. 49, Y. The posterior myelonal columns which they

49

Macromyelon, posterior or dorsal aspect, with section of
cerebellum. Infant, nat. size.

Y

' teretial tracts,' ib. A, F, bounding the sides of the fissure, called 'calamus scriptorius,' at the floor of the expanded macromyelonal canal called fourth ventricle.' This is over-arched by the cerebellum, here bisected, and one half reflected at R; the peduncle or crus' of the opposite half being shown at U. The thin layer roofing the ventricle anterior to the crus is called ' valve of Vieussens,' B.

50

P

Sections of the macromyelon, as at fig. 50, show the form of the grey matter, called 'corpus dentatum,' of the olives, o o, and the relative position of the enlarging columns. Those on each side the fissure A, are the prepyramids; those on each side the fissure P, are the post-pyramids; the lateral or restiform tracts intervene between them and the olivary tracts, o.

0

Transverse sections of parts marked x and Y

the macromyelon, at the

In the Monotremes the macromyelon is large in proportion to the rest of the brain, but the 'pons' bears relation to the cerebellum in its smallness. The prepyramids, figs. 51 and 52, a, are long, narrow, flat, and contract as they approach the pons, especially in the Ornithorhynchus; the olives, fig. 51, a, fig. 52, b, are also long and flat, but expand as they approach the pons, and are crossed, before reaching it, by the trapezoid' homologues of the arciform' fibres in Man. The distinction between the olivary and pre-restiform tracts is less marked. The grey matter

fig. 49. Man, nat. size